The Characteristic Aroma Compounds of GABA Sun-Dried Green Tea and Raw Pu-Erh Tea Determined by Headspace Solid-Phase Microextraction Gas Chromatography–Mass Spectrometry and Relative Odor Activity Value

We aim to improve the product quality of GABA raw Pu-erh tea during development and processing. In this study, headspace solid-phase microextraction gas chromatography–mass spectrometry technology combined with relative odor activity evaluations was used to compare the volatile compounds of GABA sun-dried green tea and GABA raw Pu-erh tea. Sensory evaluation showed a higher aroma score of GABA raw Pu-erh tea than that of GABA sun-dried green tea, with significant differences in aroma type and purity. A total of 147 volatile compounds of 13 categories were detected, which differed in composition and quantity between the two teas. 2-Buten-1-one,1-(2,6,6-trimethyl-1,3-cyclohexadien-1-yl)-,(E)- and beta.-myrcene largely contributed to the aroma formation of both teas. Five volatile compounds were screened as potential markers for tea aroma. Metabolic pathway analysis showed that monoterpenoid biosynthesis may be beneficial to the formation of flowery and fruity aromas in the teas. We suggest that the findings of this study may provide important guidance for the processing and optimization of GABA tea.


Introduction
Yunnan, China, is the origin center of the tea tree, contributing to a culture characterized by tea drinking for more than 1000 years [1].Pu-erh tea is a traditional and popular type of tea with a long history in Yunnan.According to the processing type and quality characteristics, Pu-erh tea can be divided into raw and ripe forms [2].In recent years, Pu-erh tea has become increasingly popular with consumers because of its unique health benefits and quality characteristics [3].Raw Pu-erh tea is a tightly pressed tea that is prepared by steaming and drying Yunnan big-leaf sun-dried green tea.Raw Pu-erh tea has the quality characteristics of a pure and lasting aroma, green and yellow clear soup color, and strong and sweet taste [4].The aroma type of raw Pu-erh tea is mainly clean and refreshing, accompanied by nutty, grassy, and fatty aromas.The composition of volatile substances is similar to that of sun-dried green tea, albeit with some distinct differences [5,6].After dry storage, the aroma of raw Pu-erh tea gradually changes to a woody, sweet, and unique aging aroma [7].With the goal of developing raw Pu-erh tea products with unique flavor, many recent studies have explored the influence of traditional processing technology on the flavor of the tea.Fan et al. collected samples at different processing stages for determination of volatile compounds (VCs), showing that deactivation and autoclaving were the key processing steps to form the flavor substances of raw Pu-erh tea.Deactivation significantly increased the relative content of aldehydes and significantly reduced the relative content of olefins.Autoclaving further reduced the contents of aldehydes and esters and significantly increased the contents of nitrogen-containing compounds and ketones.Therefore, exploring appropriate processing parameters may offer new methods to regulate the flavor of raw Pu-erh tea [8].Feng et al. found that the VCs in raw Pu-erh tea were mainly alcohols, terpenes, aldehydes, and ketones.Through aroma recombination experiments, it was determined that light, fat, flower, and fruit aromas were the characteristic aroma attributes of raw Pu-erh tea, providing a theoretical basis for processing different flavors of raw Pu-erh tea [9].
In 1987, Tsushida and colleagues discovered the method of gamma-aminobutyric acid (GABA) enrichment in tea, leading to the successful development of GABA teas (with a GABA content above 1.5 mg/g tea) [10,11].Since the content of GABA in ordinary tea is generally low, nitrogen-filled anaerobic methods are usually used to enrich GABA in tea.Under anaerobic conditions, GABA in tea is mainly enriched by GABA shunt and polyamine degradation [12].GABA tea has also been favored by the public because of its unique flavor and pharmacological effects.Zhen found that the flavor of GABA green tea processed by an anaerobic method changed significantly compared with that of ordinary green tea, resulting in an obvious sweet and sour fruit aroma, accompanied by increased contents of methylphenol, methyl myristate, methyl laurate, and methyl palmitate [13].Li et al. found that GABA white tea processed by an anaerobic method exhibited a strong floral and fruity aroma, which was mainly due to 2-heptanol [14].Our previous study demonstrated that the GABA content in the fresh leaves of Yunnan big-leaf tea trees can be effectively accumulated after 6 h of anaerobic treatment.The processed GABA sun-dried green tea and GABA raw Pu-erh tea products exhibited unique flavor qualities with certain promotion value [15].
To date, more than 700 VCs have been identified in tea, but only a small number of VCs have been confirmed to contribute to the formation of the characteristic tea flavor [16].The formation of the characteristic flavor of tea is not only affected by the concentration and taste but is also the result of the interaction between the constituent compounds and is closely related to the olfactory threshold (OT) [17].Therefore, determination of the odor activity value (OAV) and relative odor activity value (ROAV) is considered to be an important method for screening and forming characteristic aroma compounds [18,19].In addition, headspace solid-phase microextraction (HS-SPME) technology, gas chromatography-mass spectrometry (GC-MS) technology, gas chromatography-olfactometry technology, electronic nose technology, sensory evaluation, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, aroma recombination and omission experiments, and other methods [20,21] are commonly used for the screening, identification, and testing of characteristic aroma compounds in food research.Some of these techniques have also been applied to the analysis of tea.For example, Hong et al. used HS-SPME-GC-MS technology combined with sensory evaluation results and then applied OAV and multivariate statistical analysis to explore the characteristic aroma compounds of Chinese yellow tea with different aroma types.They identified 8, 14, 7, and 18 VCs in yellow tea with flowery, high-fired, fresh, and corny aromas, respectively, which could be used as the characteristic aroma compounds of yellow tea with different aroma types [22].
In this study, sensory evaluation, HS-SPME-GC-MS combined with ROAV, and multivariate analysis were used to compare and analyze the VCs of GABA sun-dried green tea and GABA raw Pu-erh tea.The key VCs of aroma formation and the main metabolic pathways of biosynthesis in GABA sun-dried green tea and GABA raw Pu-erh tea were clar-ified.These results are expected to provide a theoretical basis for optimizing the processing technology of GABA sun-dried green tea and GABA raw Pu-erh tea.

Tea Samples
Four groups of samples, including fresh tea leaves as a control (CK), anaerobicprocessed tea leaves (CA), GABA sun-dried green tea leaves (SGT), and GABA raw Pu-erh tea leaves (PRT), were prepared and compared.Three replicates were established for each group.In July 2021, all samples (one bud, two or three leaves of the same tea variety Camellia assamica vs. Yun kang 10) were picked and processed in the tea garden of Dalishu Tea Factory in Yunlong County, Dali, China.
In the CK group, the fresh leaves of tea plants were deactivated by steam at 100 • C for 5 min and then dried at 45 • C for 2 h.In the CA group, the fresh leaves of tea plants were placed in a special machine filled with N 2 for anaerobic treatment for 6 h, vaporized at 100 • C for 5 min, and then dried at 45 • C for 2 h.In the SGT group, the fresh leaves of the tea plant were air-dried for 2 h and then placed in a special machine filled with N 2 for anaerobic treatment for 6 h, followed by steam deactivation at 100 • C for 3 min, rolling for 45 min, and finally dried by sunshine for 8 h.In the PRT group, the fresh leaves of the tea plant were aired for 2 h, placed in a special machine filled with N 2 for anaerobic treatment for 6 h, and then vaporized at 100 • C for 3 min.After rolling for 45 min, the tea leaves were dried by sunshine for 8 h, autoclaved into a cake type, cooled to room temperature, dried for 3 h, and finally dried at 60 • C for 1 h.The manufacturing procedures are specified in the Chinese national standard procedure and are shown in Figure 1.
In this study, sensory evaluation, HS-SPME-GC-MS combined with ROAV, and multivariate analysis were used to compare and analyze the VCs of GABA sun-dried green tea and GABA raw Pu-erh tea.The key VCs of aroma formation and the main metabolic pathways of biosynthesis in GABA sun-dried green tea and GABA raw Pu-erh tea were clarified.These results are expected to provide a theoretical basis for optimizing the processing technology of GABA sun-dried green tea and GABA raw Pu-erh tea.

Tea Samples
Four groups of samples, including fresh tea leaves as a control (CK), anaerobic-processed tea leaves (CA), GABA sun-dried green tea leaves (SGT), and GABA raw Pu-erh tea leaves (PRT), were prepared and compared.Three replicates were established for each group.In July 2021, all samples (one bud, two or three leaves of the same tea variety Camellia assamica vs. Yun kang 10) were picked and processed in the tea garden of Dalishu Tea Factory in Yunlong County, Dali, China.
In the CK group, the fresh leaves of tea plants were deactivated by steam at 100 °C for 5 min and then dried at 45 °C for 2 h.In the CA group, the fresh leaves of tea plants were placed in a special machine filled with N2 for anaerobic treatment for 6 h, vaporized at 100 °C for 5 min, and then dried at 45 °C for 2 h.In the SGT group, the fresh leaves of the tea plant were air-dried for 2 h and then placed in a special machine filled with N2 for anaerobic treatment for 6 h, followed by steam deactivation at 100 °C for 3 min, rolling for 45 min, and finally dried by sunshine for 8 h.In the PRT group, the fresh leaves of the tea plant were aired for 2 h, placed in a special machine filled with N2 for anaerobic treatment for 6 h, and then vaporized at 100 °C for 3 min.After rolling for 45 min, the tea leaves were dried by sunshine for 8 h, autoclaved into a cake type, cooled to room temperature, dried for 3 h, and finally dried at 60 °C for 1 h.The manufacturing procedures are specified in the Chinese national standard procedure and are shown in Figure 1.

Chemicals
N-hexane (chromatographical purity) was purchased from Merk (Darmstadt, Germany).NaCl (analytical purity) was obtained from China National Pharmaceutical Group

Sensory Panel Evaluation of Teas
Sensory evaluation of tea samples was carried out according to Chinese national standard procedure [23].The members of the tea review team were all from the Tea College of Yunnan Agricultural University.The team included 10 trained reviewers (five females and five males with an average age of 30 years and professional experience of 5-35 years) who evaluated the appearance and quality of tea samples according to the Chinese national standard procedure.The score was calculated by the method of percentage weighting.First, the tea evaluation terms were discussed, and then the sensory evaluation was carried out in the special tea review room at room temperature (25 • C).The tea samples were delivered in random order to ensure that the team members did not know the sample number [24].Then, 100 g samples were randomly selected to be evaluated and have their appearance scored (A).Next, 3 g of each tea sample was taken and brewed with 150 mL of boiling water.After 5 min, the beverage was used to evaluate and score the internal quality, including aroma (B), beverage color (C), taste (D), and leaf base (E).Finally, the opinions of the group members were summarized, and the quality of tea samples was quantified according to the following formulas:

Sample Preparation
All sample preparation and detection methods were improved by referring to the methods in the existing papers [25].Materials were harvested, weighed, immediately frozen in liquid nitrogen, and stored at -80 • C until needed.Samples were ground to a powder in liquid nitrogen.One gram of the powder was immediately transferred to a 20 mL headspace vial (Agilent, Palo Alto, CA, USA) containing NaCl saturated solution to inhibit any enzyme reaction.The vials were sealed using crimp-top caps with TFE-silicone headspace septa (Agilent).At the time of SPME analysis, each vial was placed in a 100 • C water bath for 5 min, and then a 120 µM divinylbenzene/carboxen/polydimethylsilioxan fiber (Agilent) was exposed to the headspace of the sample for 15 min at 100 • C.

GC-MS Conditions
After sampling, desorption of the VCs from the fiber coating was carried out in the injection port of the GC apparatus (Model 8890; Agilent) at 250 • C for 5 min in splitless mode.The identification and quantification of VCs were carried out using an Agilent Model 8890 GC and a 5977B mass spectrometer (Agilent), equipped with a 30 m × 0.25 mm × 0.25 µM DB-5MS (5% phenyl-polymethylsiloxane) capillary column.Helium was used as the carrier gas at a linear velocity of 1.2 mL/min.The injector temperature was kept at 250 • C, and the detector temperature was maintained at 280 • C. The oven temperature was programmed from 40 • C (3.5 min), increasing at 10 • C/min to 100 • C, at 7 • C/min to 180 • C, at 25 • C/min to 280 • C, and held for 5 min.Mass spectra were recorded in electron impact ionization mode at 70 eV.The quadrupole mass detector, ion source, and transfer line temperatures were, respectively, set at 150 • C, 230 • C, and 280 • C. Mass spectra were scanned in the m/z range of 50-500 amu at 1 s intervals.Identification of VCs was achieved by comparing the mass spectra with the data system library (MWGC or NIST) and linear retention index.

ROAV Calculation
Based on the relative content of aroma compounds and the threshold value of each aroma component in water, the ROAV calculation formula is determined as follows: where C n represents the relative content of any VC (%), C max represents the relative content of the largest VC (%), T n represents the OT of any VC, and T max represents the maximum OT of VCs; the greater the ROAV value, the greater the contribution value of the compounds to the aroma.VCs with ROAV > 1 can be considered to have an important contribution to the composition of the aroma, whereas VCs with 0.1 < ROAV < 1 can be considered to have a modification effect on the composition of the aroma [26].

KEGG Pathway Analysis
To explore the biosynthesis pathways of aroma compounds during the processing of tea samples in the SGT and PRT groups, the detected VCs were compared with the known compounds in the KEGG database (https://www.kegg.jp/kegg/pathway.html(accessed on 15 August 2023)), and metabolic pathway analysis was performed using MBRole 2.0 (http://csbg.cnb.csic.es/mbrole2/(accessed on 15 August 2023)) [27].

Statistical Analysis
All data were measured three times in parallel, and the results are expressed as mean ± standard deviation.SPSS v.26 was used for analysis of variance, and Tukey's post-hoc test.p < 0.05 was used as the standard for judging a significant difference between groups.Metware Cloud was used for principal component analysis (PCA) and orthogonal partial least-squares discriminant analysis (OPLS-DA).Draw Venn and Origin 2022 were used for drawing the clustering heat map and other data analysis and mapping.

Sensory Evaluation of Aroma Characteristics of GABA Sun-Dried Green Tea and GABA Raw Pu-Erh Tea
The sensory evaluation method, as the most basic tea quality evaluation method, is based on people's intuitive feelings.Nevertheless, this method remains irreplaceable and is widely used for the evaluation of all tea types [28,29].The evaluation of aroma factors in tea sensory evaluation usually includes four aspects: aroma type, concentration, purity, and persistence [30].
As shown in Table 1, the aroma characteristics of the SGT and PRT were significantly different.The aroma of SGT was fragrant, high, pure, and lasting, and the flowery aroma was rich, while the fruity aroma was weak.The aroma of the PRT was purer and longerlasting and had flowery, fruity, and sweet characteristics.The aroma factor score and total scores of PRT were higher than those of SGT.Since a previous study showed that the processing type (autoclaved) can enhance the flavor of raw Pu-erh tea [31], and the autoclaved process is a unique part of the PRT, the autoclaving process may be the cause of the difference in aroma characteristics between the two tea samples.Using HS-SPME-GC-MS technology to detect GABA sun-dried green tea and GABA raw Pu-erh tea, a total of 148 VCs were identified (Table 2).Among them, 145, 145, and 143 VCs were detected in SGT, PRT, and CK samples, respectively, and all 148 VCs could be detected in CA samples.After comparing the types of VCs between tea samples, it was found that after anaerobic treatment of CK, a total of five VCs were newly generated, including 9,12-octadecadienoic acid, (Z,Z)-, methyl ester; 9,12,15-octadecatrienoic acid, (Z,Z,Z)-; (Z)-9,17-octadecadienal; linoleic acid ethyl ester; and (E)-2-hexenoic acid, butyl ester.The first four VCs were also detected in both SGT and PRT, and their contents increased.Therefore, further study is needed to determine whether these four compounds affect the formation of SGT and PRT aroma characteristics.However, the last VC was only detected in CA, indicating that anaerobic treatment can increase the type of VCs formed.After SGT was autoclaved into PRT, two new VCs were generated, 2-(1,1-dimethylethyl)-6-(1-methylethyl) phenol and 4H-pyran-4-one,2,3-dihydro-3,5-dihydroxy-6-methyl-.Further analysis showed that tea samples with different treatments could be clustered, indicating that the data were representative [32], and there were significant differences in the contents of VCs among different tea samples (Figure S1).The results of PCA showed that the four groups of tea samples could be clustered separately with obvious separation between the groups (Figure 2a).In addition, there were also some differences in the types of VCs among different tea samples (Figure 2b).
PRT, CK, and CA samples.Compared with the SGT, the contents of 18 terpenes in the PRT increased significantly, and the total content of terpenes increased by 1.4 times, indicating that the autoclave technology contributed to the formation of terpenes in PRT.A variety of terpenes have been found and confirmed to be the characteristic aroma compounds in tea, such as linalool with a sweet, woody, flowery, and fruity aroma [36]; [2H3]-beta.-ionone with a woody and violet flowery aroma, accompanied by a fruity aroma [37]; Lalpha.-terpineol with a flowery and light grassy aroma [38]; and geraniol with a sweet and light rose aroma [39].The aroma characteristics of these terpenes can help to better explain the aroma type of PRT.In addition, most of the increased terpenes have a relatively low OT [40], which can be used as a basis for further verification of whether these terpenes contribute to the formation of aroma in PRT.
OPLS-DA is generally used to distinguish the differences of variables between two groups.In this study, OPLS-DA was used to distinguish and analyze the different VCs between SGT and PRT samples (Figure 2c).A total of 68 differential VCs between the SGT and the PRT were screened (variable importance in projection (VIP) > 1, p < 0.05, fold change > 0).By comparing the SGT with the PRT (Figure 2d        Represents that the post-test has been carried out, different lowercase letters in the same line indicate significant difference (p < 0.05), and '-' indicates that it is not detected.Each tea sample was measured in parallel 3 times, and all data were expressed as mean value ± SD.
According to the isoprene rules in organisms, the carbon units are condensed to form precursors for the synthesis of terpenes, which have basic functions in the growth and development of organisms and continue to form more terpenes through metabolic pathways [33].According to existing research, terpenes are important contributors to the VCs of green tea, black tea, Pu-erh tea, and other teas, but they are rarely reported as the first-class compound category [34,35].Therefore, the molecular configuration and metabolic pathway of terpenes can be used as an important direction for the study of characteristic aroma compounds in tea in the future.In this study, 26 terpenes were detected in SGT, PRT, CK, and CA samples.Compared with the SGT, the contents of 18 terpenes in the PRT increased significantly, and the total content of terpenes increased by 1.4 times, indicating that the autoclave technology contributed to the formation of terpenes in PRT.A variety of terpenes have been found and confirmed to be the characteristic aroma compounds in tea, such as linalool with a sweet, woody, flowery, and fruity aroma [36]; [2H3]-beta.-ionone with a woody and violet flowery aroma, accompanied by a fruity aroma [37]; L-alpha.terpineol with a flowery and light grassy aroma [38]; and geraniol with a sweet and light rose aroma [39].The aroma characteristics of these terpenes can help to better explain the aroma type of PRT.In addition, most of the increased terpenes have a relatively low OT [40], which can be used as a basis for further verification of whether these terpenes contribute to the formation of aroma in PRT.

Key Active Compounds of GABA Sun-Dried Green Tea and GABA Raw Pu-Erh Tea Aroma
The types of VCs in SGT and PRT are very complex, and many of the VCs identified are odorless or have low odor activity with no contribution to or minimal effect on the overall aroma formation of tea samples.Therefore, the key active compounds of SGT and PRT aroma formation were screened to reveal the key role of VCs in the formation of tea aroma.A total of 14 VCs were screened based on ROAV and aroma types (Table 3).In the SGT, there were six VCs with ROAV > 1 and eight compounds with 0.1 < ROAV < 1.In the PRT, there were five VCs with ROAV > 1 and eight compounds with 0.1 < ROAV < 1.The Pearson correlation circle showed that there was a weak correlation between 2-buten-1one,1-(2,6,6-trimethyl-1,3-cyclohexadien-1-yl)-, (E)-and other compounds in PRT, which was more prominent (Figure 3a).In SGT, 1-octen-3-ol and D-limonene showed a weak correlation with other compounds (Figure 3b).Further analysis of ROAV by construction of the cluster heat map showed that the key active compounds in SGT and PRT were significantly different (Figure 3c).Each tea sample was measured in parallel for 3 times, and all data were expressed as mean value ± SD.
The production of potential markers is usually related to the variety, processing technology, or unique active ingredients.The identification of potential markers of tea can help tea quality control or test repeatability [46].According to previous studies, potential marker aroma compounds may be formed during tea storage or processing, and even biological pathways such as oxidation and degradation dominated by environmental factors may also promote the formation of potential marker aroma compounds [47,48].According to the five potential marker aroma compounds found in this study, we believe that it is likely to be due to anaerobic treatment.Potential marker aroma compounds have practical significance for explaining the aroma results of this study and will provide a reference for future research on aroma compounds of GABA tea.

Metabolic Evolution Pathway of Main Aroma Compounds
The main VCs in SGT and PRT were terpenes and alcohols, which was similar to the results of previous studies [49].KEGG pathway analysis was used to further explore the metabolic pathways for the formation of key aroma compounds in SGT and PRT.The results showed that the formation of major VCs mainly involves monoterpenoid biosynthesis, biosynthesis of phytochemical compounds, and amino acid metabolism (Figure 4).
Combined with the characteristic aroma compounds map and metabolic pathway map of PRT (Figure 5), it can be found that terpenes in SGT and PRT are mainly formed through monoterpenoid biosynthesis.Most of the terpenes in tea have woody, flowery, and fruity aromas, which can better coordinate the aroma of SGT and PRT.According to a previous study, the product of glycoside hydrolysis, geranyl diphosphate, is an important precursor of terpenoids [50].Hydrolases hydrolyze glycoside to form geranyl diphosphate in the process of water loss and anaerobic digestion of SGT and PRT, which leads to the formation of various terpenes in the tea through the enzymatic action of corresponding synthases.In addition, linalool forms geraniol under the action of isomerase.Indole is mainly formed through the amino acid metabolism pathway, and anethole is mainly formed through the biosynthesis of phytochemical compounds, which has a certain contribution to SGT aroma; on the one hand, it enriches the SGT aroma type, and on the other hand, it affects the purity of the SGT aroma.However, the contribution of indole to the PRT aroma was not significant.A previous study found that high-temperature drying will lead to the degradation of a large number of amino acids in black tea [51].In the process of SGT deactivation and autoclaving, tryptophan, as a precursor for the synthesis of indole, is greatly degraded, thus reducing the contribution of indole to the PRT aroma.

Conclusions
In summary, the main VCs in SGT and PRT were formed by monoterpenoid biosynthesis, biosynthesis of phytochemical compounds, and amino acid metabolism during processing.We believe that terpenes play an important role in promoting the aroma of SGT and PRT, mainly including linalool, geraniol, 2-buten-1-one,1-(2,6,6-trimethyl-1,3-cyclohexadien-1-yl)-,(E)-, and beta.-myrcene,resulting in flowery and fruity aroma characteristics.Compared with PRT, 1-octen-3-ol, indole, and anethole may contribute to the more impure aroma of SGT, which may be related to the autoclave processing technology.In addition, the qualitative and quantitative results of VCs were quite different between

Figure 1 .
Figure 1.The process of GABA sun-dried green tea and GABA raw Pu-erh tea manufacturing.

Figure 1 .
Figure 1.The process of GABA sun-dried green tea and GABA raw Pu-erh tea manufacturing.

Figure 2 .
Figure 2. Multivariate statistical analysis of volatile organic compounds in the processing of GABA sun-dried green tea and GABA raw Pu-erh tea.(a) PCA model score scatter plot (total).(b) Venn diagram of volatile organic compounds.(c) Scatter plot of scores of OPLS-DA model (SGT vs. PRT).(d)Volcano plot of differential compounds (SGT vs. PRT).CK refers to fresh tea leaves, CA refers to anaerobic tea leaves, SGT refers to GABA sun-dried green tea, and PRT refers to GABA raw Pu-erh tea.

Figure 2 .
Figure 2. Multivariate statistical analysis of volatile organic compounds in the processing of GABA sun-dried green tea and GABA raw Pu-erh tea.(a) PCA model score scatter plot (total).(b) Venn diagram of volatile organic compounds.(c) Scatter plot of scores of OPLS-DA model (SGT vs. PRT).(d) Volcano plot of differential compounds (SGT vs. PRT).CK refers to fresh tea leaves, CA refers to anaerobic tea leaves, SGT refers to GABA sun-dried green tea, and PRT refers to GABA raw Pu-erh tea.

Figure 3 .
Figure 3. Based on the ROAV value, the correlation statistics of the characteristic aroma compounds of tea samples were carried out.SGT refers to GABA sun-dried green tea, and PRT refers to GABA raw Pu-erh tea.(a) Pearson correlation circle of characteristic aroma compounds of PRT.(b) Pearson correlation circle of characteristic aroma compounds of SGT.(c) Heat map of hierarchical clustering of relative odor activity value.

Figure 3 .
Figure 3. Based on the ROAV value, the correlation statistics of the characteristic aroma compounds of tea samples were carried out.SGT refers to GABA sun-dried green tea, and PRT refers to GABA raw Pu-erh tea.(a) Pearson correlation circle of characteristic aroma compounds of PRT.(b) Pearson correlation circle of characteristic aroma compounds of SGT.(c) Heat map of hierarchical clustering of relative odor activity value.

Figure 4 .
Figure 4. Metabolic evolution pathway of main flavor compounds in GABA sun-dried green tea and GABA raw Pu-erh tea.

Figure 5 .
Figure 5. Characteristic aroma compounds and characteristic aroma in GABA raw Pu-erh tea.

Figure 4 .
Figure 4. Metabolic evolution pathway of main flavor compounds in GABA sun-dried green tea and GABA raw Pu-erh tea.

Figure 4 .
Figure 4. Metabolic evolution pathway of main flavor compounds in GABA sun-dried green tea and GABA raw Pu-erh tea.

Figure 5 .
Figure 5. Characteristic aroma compounds and characteristic aroma in GABA raw Pu-erh tea.

Figure 5 .
Figure 5. Characteristic aroma compounds and characteristic aroma in GABA raw Pu-erh tea.

Table 1 .
Sensory evaluation results of GABA sun-dried green tea and GABA raw Pu-erh tea.
3.2.Quantitative and Qualitative Analysis of VCs in GABA Sun-Dried Green Tea and GABA Raw Pu-Erh Tea

Table 2 .
Volatile compound composition and absolute contents.

Table 3 .
ROAV and aroma types of key aroma compounds.Each tea sample was measured in parallel for 3 times, and all data were expressed as mean value ± SD.